A family of about 50 enzymes with a molecular mass of 21 kDa that are distantly related to the alpha-subunit of heterotrimeric G-protein GTPase (EC 3.6.5.1). They are involved in cell-growth regulation (Ras subfamily), membrane vesicle traffic and uncoating (Rab and ARF subfamilies), nuclear protein import (Ran subfamily) and organization of the cytoskeleton (Rho and Rac subfamilies).